This invention relates to a process for removing impurities from cellulose ether compositions.
Cellulose ethers are a wide variety of polymers derived from cellulose. Such polymers can be water-soluble, organo-soluble or both. These polymers are useful in a wide variety of applications, for example, as thickeners in aqueous or organic liquids, film formers, and binders in the pharmaceutical and food fields.
Typically, cellulose ethers are prepared by reacting alkali cellulose with one or more etherifying agents. The etherifying agents are generally alkylhalides, alkylene oxides, halocarboxylic acids or dialkyl sulfates. The etherifying agents are highly reactive, and can form impurities upon reacting with each other, water, and the alkali on the cellulose. The impurities formed in these side reactions must be removed before the cellulose ether can be employed in end uses, for example, in the pharmaceutical and food fields.
In view of the fact that many water-soluble cellulose ethers are insoluble in hot water, the impurities in the final product can be removed by washing the cellulose ether product in hot water at a temperature above the gel point, i.e., the temperature at which the cellulose ether precipitates. Typically, such a procedure is performed by spreading the cellulose ether product on a large filter, and then spraying the product with hot water. However, an amount of the cellulose ether remains soluble in the hot water. Moreover, for those cellulose ethers which have gel points close to the boiling point of water at standard pressure, a larger portion of the cellulose ether remains soluble. The portion of the cellulose ether which remains soluble is washed away in the hot water wash causing product loss. For example, it is typical to lose about 20 to about 30 percent of low viscosity hydroxypropyl methylcellulose and about 5 to about 9 percent of high viscosity hydroxypropyl methylcellulose. The low viscosity species experience higher losses because a larger portion is soluble in water at any given temperature.
The problem of product loss in the hot water washing process is aggravated by the fact that a large quantity of water is required to remove the desired amount of impurities. Typically, from about 7 to about 20 parts of water are required to remove one part of the impurities. This large amount of water removes a correspondingly large amount of cellulose ether product along with the impurities. Additionally, when this large amount of water containing the impurities and products is discharged, large costs are incurred to decontaminate it.
Recently, in U.S. Pat. No. 4,404,370 it has been disclosed that cellulose ethers can be purified in a counter-current process. In such a process, a suspension of cellulose ether is dropped onto a conveyor belt, and washed continuously with a water/alcohol purifying agent. The purifying agent is sprayed onto the cellulose ether from a number of ports, and filtered off. Some of the filtered purifying agent is recycled, and used as the purifying agent at a different port. Such a process has the advantage of efficient use of the purifying agent, but does not provide a means for controlling other process parameters such as temperature and pressure, and requires the addition of a purifying agent which is different from the suspension agent.
In view of the disadvantages of known methods, it is desirable to provide a process for removing impurities from cellulose ether compositions in an efficient, versatile wash system, in which substantial amounts of the impurities are removed without experiencing substantial cellulose ether product losses.